The present invention generally relates to subterranean well completion apparatus and, in a preferred embodiment thereof, more particularly relates to subsea subterranean well completion apparatus which is provided with a specially designed flow assurance system that prevents the formation of, or removes, hydrates and wax deposits in the production tubing string without the previous necessity of injecting chemicals into the tubing string.
In deep water well completions, in which the production riser portion of the completion often extends through rather frigid water above the sea bed, potential plugging or restriction of the tubing string due to the formation of hydrates or wax deposits is a major concern. This is especially critical in the production riser interval near the sea floor where ambient temperatures are the coldest. To mitigate this potential problem, it has been common practice to use a subsurface chemical injection system to inject inhibitors and other chemicals into the production tubing string to prevent the formation of the hydrates and/or wax deposits. Examples of such a chemical injection system are schematically depicted in FIGS. 1 and 3 of U.S. Pat. No. 5,875,852 which is hereby incorporated by reference herein in its entirety.
The use of chemical injection systems to inhibit hydrate and/or wax deposit plugging in a production tubing string carries with it several well known problems, limitations and disadvantages. For example, chemical injection systems of this type tend to be quite complex and expensive, entailing various control lines, injector apparatus and related controls. Additionally, associated chemical handling equipment is required at the surface which adds expense and complexity to the completion""s surface equipment. Moreover, the chemicals typically injected into the production flow to inhibit hydrate and/or wax plugging or restriction of the production tubing string are typically flammable and/or toxic in nature, thus adding a safety risk to the overall production process.
It can thus be seen from the foregoing that it would be desirable to incorporate in a well completion system of the type generally described a flow assurance system for preventing or removing hydrate/wax plugging or restriction without the use of the injection of chemicals into the production flow. It is to this goal that the present invention is directed.
In carrying out principles of the present invention, in accordance with a preferred embodiment thereof, a subterranean well completion, representatively an underwater subterranean well completion, is provided with a flow assurance system which is operative to prevent or remove hydrate/wax plugging or restriction of production fluid disposed in a tubing string portion of the completion through which the production fluid may be flowed to the surface. Uniquely, the flow assurance system is operative without the conventional necessity of injecting plug-inhibiting chemicals into the interior of the tubing string.
From a broad perspective, the flow assurance system forms a flow path through which a heated fluid may be recirculated, within the space in the completion apparatus that surrounds the tubing string, in a manner causing the heated fluid to flow along the outer side of the tubing string, thereby transferring heat thereto, without interiorly traversing the tubing string. The flow path may also be used to recirculate other types of fluids in this manner including, for example, a corrosion inhibiting fluid or an insulating fluid.
In an illustrated underwater embodiment of the well completion apparatus which incorporates the flow assurance system, the completion apparatus includes a wellbore extending through the earth through a subterranean production formation, and a production riser extending upwardly from the wellbore through the water. An outer tubing string extends through the production riser and wellbore and forms therewith an outer annular space, and an inner tubing string extends through the outer tubing string and forms therewith an inner annular space. Within the outer tubing string an annular seal structure circumscribes and seals a downhole end portion of the inner string within the outer tubing string. Fluid from the formation is flowable to the surface sequentially through (1) a longitudinal portion of the outer tubing string downhole of the seal structure and (2) the inner tubing string.
An annular packer, set at a subterranean depth at which the temperature is too high to create hydrate or wax deposits in production fluid within the inner tubing string during pre-production or shut-in periods of the completion apparatus, circumscribes the outer tubing string and defines a downhole end of the outer annular space. A sidewall opening is formed in the outer tubing string uphole of the packer and communicates the outer and inner annular spaces.
The outer and inner annular spaces, and the outer tubing string sidewall opening that communicates them, define a flow path through which a selected fluid, such as a corrosion inhibiting fluid or a heated fluid for inhibiting wax and/or hydrate formation in production fluid within the inner tubing string portion above the packer, may be recirculated and caused to flow along the outer side of the inner tubing string without interiorly traversing it. Representatively, a valve, preferably a surface operated sliding sleeve valve, is carried by the outer tubing string and is operative to selectively cover and uncover the sidewall opening therein.